1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, it relates to a surface acoustic wave device which comprises a piezoelectric substrate and electrodes of aluminum provided on the piezoelectric substrate.
2. Description of the Background Art
In recent years, widely employed is a surface acoustic wave device such as a filter, a resonator or the like using surface acoustic waves, which may be hereinafter referred to as SAW.
Such a surface acoustic wave device generally comprises a piezoelectric substrate, which is provided on its surface with interdigital transducers and/or grating reflectors formed by metal strips.
The piezoelectric substrate is prepared from a single crystal material such as quartz crystal, lithium tantalate (LiTaO.sub.3), lithium niobate (LiNbO.sub.3) or lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7), or a ZnO/Al.sub.2 O.sub.3 material formed by a substrate of sapphire (Al.sub.2 O.sub.3) and a film of zinc oxide (ZnO) provided thereon.
The aforementioned interdigital transducers and grating reflectors are generally prepared from aluminum, which is easy to photolithograph and has a small electrode loading mass effect due to its small specific gravity, and high conductivity.
However, it has been recognized that, when a signal of a high voltage level is applied to a surface acoustic wave device such as a SAW filter or resonator, the aluminum electrodes are exposed to strong stress by surface acoustic waves, to cause migration. Such migration caused by stress is called stressmigration, to be distinguished from electromigration. The stressmigration leads to an electric short, increase in insertion loss, reduction in quality factor (Q) of the resonator, and the like. Such stressmigration easily occurs as the frequency is increased, to cause a significant problem particularly in a surface acoustic wave device which is used in a high frequency range.
As to a resonator, in particular, it is necessary to increase the gain of an oscillation circuit for applying a signal of an excessive voltage level, in order to cause stable oscillation. In a resonator having reflectors which are provided on both sides of one or some transducers, surface waves are confined between the reflectors and standing waves are applied to the transducers and reflectors, to apply strong stress thereto. Thus, stressmigration is particularly easily caused in the resonator. To this end, a conventional SAW resonator has been unavoidably driven at a low voltage level, in order to suppress such stressmigration to the minimum. Thus, it has been impossible to increase the C/N ratio (carrier-to-noise ratio) or to suppress SSB (Single-Side-Band) phase noises.
As to a SAW filter, stressmigration particularly easily occurs in a transmission filter, which receives a signal of a high voltage level.
In order to prevent the aforementioned stressmigration, a small amount of Cu, Ti, Ni, Mg, Pd or the like is added to the electrode material of aluminum. However, no satisfactory effect has been attained by such a countermeasure.
A substrate of Y-cut (LST-cut) quartz crystal rotating at an angle of about 105.degree. has been recently watched as a substrate for a surface acoustic wave device having excellent temperature characteristics. The surface acoustic wave device formed by such a substrate of LST-cut quartz crystal is superior in temperature characteristic to that formed by a substrate of ST-cut quartz crystal, such that leakage surface acoustic waves (LSAW) are propagated on the surface of the substrate.
However, it is known that, when such an LST-cut quartz crystal substrate is applied to a resonator, for example, its quality factor is abruptly reduced to deteriorate characteristics if the thickness of aluminum electrodes exceeds 1% of the wavelength of surface acoustic waves, although no particular problem is caused when an ST-cut quartz crystal substrate is employed even if the thickness of aluminum electrodes is about 2% of the wavelength of the surface acoustic waves. If the aluminum electrodes provided on the LST-cut crystal substrate are reduced in thickness in order to avoid such deterioration of the characteristics, apparent electrical resistivity is increased since general polycrystalline aluminum forming the electrodes has a large grain size. Thus, insertion loss is increased and the quality factor is reduced. Such disadvantages are remarkable particularly in a high-frequency range of short wavelengths.